SIGNIFICANT RESULTS FROM RESEARCH ON ARCHAEAN ROCKS OF THE DHARWAR CRATON

Srinivasan has worked on various aspects of the Archean rock formations of the Dharwar craton to understand the evolution of the Archaean crust as evidenced here. He has worked on sedimentation, paleoenvironment and climate, paleobiology, volcanism, structural history, metamorphism, geochemistry and geochronology of the supracrustal rocks of the schist belts and gneisses of the Dharwar craton. Important results under each of the above heads are as follows :

Sedimentation, paleoenvironment and paleoclimate:

He was one of the earliest to distinguish the platform, shelf and deep water environments of sedimentation in the Dharwar craton and related sedimentation and tectonics. He working with Prof. Ojakangas showed that the quartz pebble conglomerates and quartzites of the Bababudan Group were deposited in braided fluvial plains and these are interbedded with subaerial basaltic volcanic rocks. Such sedimentary association was rarely ever reported in the Archaean greenstone belts at the time when they were reported. Similar sequence was discovered later in the Uchi greenstone belt of Canada by Phil Thurston. On the basis of occurrence detrital pyrite and uraninite in the quartz pebble conglomerates of the Bababudan sequence, he suggested that during the early stages of sedimentation of the Dharwar Sequence, the atmosphere was oxygen deficient. In a detailed restudy of the conglomerates at the base of the Chitradurga Group, in association with Prof. Ojakangas, Srikantia and Hegde, Srinivasan has presented confirmatory evidences for the glaciomarine origin of these conglomerates which are characterized by diamictite-drop stone association. Geochronological studies indicate that this glaciation is 2.7 Ga old and represents second oldest glaciation event in the Archaean earth history. He has shown that during the amelioration from this glacial episode, carbonate rocks were deposited, in which there are well - developed stromatolites built by cyanobacterial algae. C and O isotopic studies of these carbonate rocks showed that they were deposited in a marine environment. Some of them still retain C and O isotopic characteritics of cap carbonate rocks. Stromatolite occurrences have been helpful in defining the palaeo seacoast and shallow shelf region in the Archean greenstone belts. Stromatolite proliferation in the Dharwar seas was probably facilitated by normal low temperature conditions as revealed by some C and O isotopic study on co-existing cherts and carbonates. Srinivasan and Ojakangas observed graded bedding and Bouma cycles in the greywackes of the Chitradurga Group indicating deposition under the influence of turbidity currents. Petrographic studies indicated that they consist of continent derived detritus as well as volcanic rock fragments as can be expected in a back arc basin. Geochemical studies of greywackes led to the evidence that the provenance had witnessed upper granodioritic crustal differentiation prior to greywacke sedimentation and that chemical weathering was very sudued in the provenance.

Palaeobiology:

In association with scientists of BSIP and his colleagues at NGRI, Srinivasan for first time discovered by scanning electron microcopy the occurrence of coccoid as well rod shaped bacteria in the banded iron formations Kudremukh area of Bababudan Group. He provided the evidences for syngenetic nature of the cyanobacterial fossil filaments found in the banded iron formation of Sandur belt. Srinivasan also worked on the C isotopic composition of graphites which showed that the graphite was formed from organic carbon. Extremely low δ C13 values in Ganacharpur graphites of Kolar schist belt suggested that there was intervention of metanogenic bacteria in the fractionation of carbon during the Archean.

Volcanism

Srinivasan was one of the earliest to suggest subaerial flood basalt volcanism for the volcanic rocks of the Bababudan Group. He observed these rocks are tholeiitic in composition. Their low K tholeiitic composition was similar to volcanics in back arc basins. By contrast, in the Chitradurga Group, he observed that submarine volcanism was dominant. He described occurrence of dacites in the volcanic assemblage. On the basis of dominanace of bimodal volcanic association in the Bababudan, Dharwar-Shimoga and Chitradurga belts, and occurrence calc-alkaline volcanic assemblages in the schist belts in the eastern part of the Dharwar craton, Srinivasan has proposed that the Dharwar schist belts in the western part evolved in back arc foreland basins, while the eastern belts evolved in magmatic arc environment.

Structural history of the Dharwar craton

Srinivasan with late Prof. Naha and Prof. D. Mukhopadhyay has carried out structural studies in the Dharwar craton. These studies have shown structural unity ( in terms of style, sequence and orientation ) throughout the Dharwar craton. The structural history in the western and eastern Dharwar schist belts, in the so called Dharwar and Sargur Group of rocks, in the Peninsular Gneiss and in the charnockites have been shown to be similar. This structural unity has been interpreted to indicate that : (1) the unconformity at the base of the Bababudan Group is not an angular unconformity; (2) the Peninsular Gneiss is a complex consisting of some basement rocks as well as syntectonic gneisses that evolved during the deformation of Dharwar and Sargur Supracrustal rocks; (3) Sargur Group consists of torn remnants of rocks from the lower sections of the Dharwar schist belts as well as some older rocks with pre-Dharwar fabric; (4) although traversed by crustal scale faults, the structural unity suggests that the Archean crust extended at least up to the Palghat –Cauvery lineament; (5) Nilgiri block is a tectonic horst of Archean granulites, not a separate cratonic block attached to the Dharwar craton. (6) With Prof. Mukhopadhyay , he observed a distinct structural break between the granulite terrain to the north and gneiss-migmatite terrain to the south of the Palaghat-Cauvery lineament in the Namakkal area of Tamil Nadu.

Metamorphism

1. Srinivasan for the first time proposed the idea of high temperature - low pressure metamorphic belt in the Dharwar craton based on discovery of andalusite in the Sandur schist belt and occurrence of andalusite as well as cordierite –hypersthene ± sillimanite assemblage along the border of the Closepet Granite. This report was fore- runner for the concept of low- and intermediate pressure metamorphic belts in the Dharwar craton proposed by Rollinson, Windley and Ramakrishnan. Parallel studies by Srinivasan as well Ramaksrishnan have led to the concept of occurrence paired metamorphic belts in the Archaean Dharwar craton, a concept that goes well with plate tectonics in convergent tectonic settings.
2. A detailed study of green garnets from Bandihalli region was undertaken by Srinivasan along with Dr. Parthasarathy and they found that these are not green garnets derived from kimberlite, but were normal grossularite as seen in calc-silicate assemblages. This study also provided for the first time evidence for continuous solid solution between uvarovite and grandite which was not known earlier.
3. Detailed investigations of Shungites from Precambrian Kondapoga area in Russia in association with Dr. Parthasarathy led to the discovery first report of natural fullerene from carbonaceous rocks. The fullerenes were formed under normal greenschist facies of metamorphism.
4. Srinivasan and Naha recognized two generations of orthopyroxenes developed during two phases of folding in the charnockites of southern Karnataka. This led to the suggestion that granulites were formed during two episodes, not one as was generally believed.

Geochemistry

Srinivasan studied Geochemistry of volcanic rocks, the quartzites, greywakes, limestones and cherts. The study on volcanic rocks indicated that Bababudan Group volcanics are essentially bimodal, where as in the Chitradurga Group there are volcanic rocks of calc-alkaline series. Calc -alkaline volcanic rocks are more abundant in the schist belts of the eastern Dharwar craton. The REE geochemistry quartzites and greywackes provided evidence for upper granodioritic crustal differentiation prior to the deposition of the Dharwar Supergroup sediments. The C and isotopic geochemistry of carbonate rocks indicated that they are marine deposits, some have cap dolomite characters , C and O isotopic study of coexisting cherts and carbonates with Dr. Das Sharma indicated deposition of carbonates in sea waters with normal temperatures of 20o or lower and that the Archaean sea water was not scalding as believed by some other workers. At present he is working in collaboration with Prof. Balakrishnan at Pondicherry University on the Sr and Nd isotopic systematics in carbonate rocks as well as in iron formation. Initial results show that iron formations are composed of both hydrothermal as well as hydrogenous types, not just hydrothermal as has been reported in recent years by various workers.

Srinivasan while supervising the Ph.D. work of Dr. Senthi Kumar studied the heat generation characteritics of the various rock formations of the Dharwar craton using U,Th and K abundances in rocks . This craton wide study showed that the Archean ganulite terrain is least heat producing and the tonalitic gneisses dominant in the western part of the Dharwar craton generate much less heat compared the granodiritic crust of the eastern Dharwar craton. He used the crustal heat production data and estimated the residual mantle heat flow in the Dharwar craton and showed that it is anomalously high in the northerm granulite terrain (north of P-C lineament) where it is 26-29mW m-2. By contrast it is low in the Dharwar granite greenstone terrane 12-19 mW- just as in other Archeangranite-greenstone provinces. Mantle metamoatism in the northern granulite terrane has been indicated.

Geochronology

Srinivasan carried out geochronological studies of the gneisses by Rb-Sr and Zircon Pb-Pb isotopic methods ( with Drs. Gopalan and Bhaskar Rao, and Russian Scientists), anorthosites (with Gopalan and Bhaskar Rao), of carbonate rocks ( with Gopalan , Zacharia and Sarangi), cherts ( with Gopalan, Sivaraman and Pantulu), of greywackes ( with Gopalan). These studies showed

1. age of the Peninsular gneiss falls into three groups, 3.3 Ga, 3Ga and 2.6 to 2.5 Ga and therefore the Peninsular Gneiss is a complex of gneisses of several generations. Geochronological studies supported the syntectonic evolution of Peninsular Gneiss which had been proposed on the basis of structural studies that had been carried out earlier with Prof. Naha and and Prof. Mukhopadhyay.
2. That the Rb-Sr and Sm-Nd ages are reset during metamorphism was demonstrated by a study of anorthosites from Holenarasipur. Where the N-S foliation related to later folding is superimposed on the anorthosites, they yield an age of 2.55-2.6Ga, where this fabric is absent the original age of emplacement of 2.9 Ga is preserved.
3. Pb-Pb dating of marbles of Sargur Group showed that they were metamorphosed Ca 2.5Ga just as the marbles in the Dharwar Supergroup. No evidence of metamorphism older than 3 Ga could be recognised
4. For the first time Rb-Sr dating of cherts from the Vanivilas carbonate sequence was attempted and it was shown that they preserve the same ~2.5 Ga metamorphic age as observed in the associated volcanics.
5. In a first attempt to date greywackes directly, a Rb-Sr isochron was generated from layers of single Bouma bed which also showed that the Rb-Sr ages are same as those of the associated volcanics.
6. With Dr. Goswami and Dr. Maibam Bidyananda he carried out Pb-Pb isotopic geochronology of the gneisses and supracrustal enclaves in them, both from the eastern and western part of the Dharwar craton. The study showed that the antiquity of the Eastern and western parts of the Dharwar craton is one and the same and the Western Dharwar craton is not older as had been proposed some other workers. Only there are more ~ 2.55 Ga granitic intrusions in the eastern part.
7. At present he is engaged in SHRIMP U-Pb geochronological studies with Dr. Sushanta Sarangi of ISM, Dhanbad, Dr. Maibam Bidyananada of Manipur University and Prof. Alan Nutaman of Australia.
8. U-Pb and Lu-Hf systematics of zircons show that the Sargur Group ( which is believed to be oldest rock sequence in south India (older than 3 Ga) is actually just as old as the 2.6 to 2.9 Ga Dharwar sequence. (Work with Maibam Bidyananda and Alex Gerdes, 2017)

Gold Mineralization

With Dr. Sarang, ISM, Dhanbad,i and his research students, Srinivasan has been researching on the source of gold mineralizing fluids in the Dharwar craton. Carbon, oxygen and Sulphur isotopic studies have led to proposition that gold deposits were formed from juvenile fluids, rather than from metamorphic hydrothermal solutions.